netbsd.compat: don't use musl's sys/cdefs.h
netbsd.compat: don't use musl's sys/cdefs.h

When building glib statically, a Meson check would fail, because the
check would interpret any warning as failure, and it would see the
warning that the musl sys/cdefs.h emits about the file being
deprecated.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs